Platinum group metals (PGM: platinum, iridium, osmium, palladium, rhodium, and ruthenium) are regarded as strategic metals. These metals are used by various industries in multiple ways including automobile, electrical and electronic, dental and medical, petroleum refining and numerous chemical industries. The major source of the platinum group metals is frequently associated with Cu-Ni deposits. These metals usually occur in conjunction with nonferrous metal sulfide ores. Another source of PGM which is becoming important, especially in the U.S., is the secondary source; namely, scrap of ceramics/glass, electrical components, and spent catalysts.
About 30 million automobiles are scrapped worldwide every year, of which more than 15 million automobiles are junked in the U.S. There are, in general, three grades of automobile catalytic converters in terms of the PGM content. Grade 1 consists of 1200 ppm of Pt, 200 ppm of Pd and 300 ppm Rh; Grade 2 consists of 1000 ppm of Pt, 200 ppm of Pd, and 100 ppm of Rh; while Grade 3 consists of 875 ppm of Pt, 250 ppm Pd and 30 ppm of Rh. On the other hand, petroleum refinery catalyst, typically contain about 3,000 ppm of platinum and 2,600 ppm of rhenium. The petroleum industry consumes about 5% of the total platinum and 30% of the total rhenium demand in the U.S.
PGM are traditionally recovered by aqua regia, HCI/HNO.sub.3 or HCI/Cl.sub.2. Because platinum-group metals are very inert, their extraction is very expensive. For example, the extraction of these metals from automobile catalysts is known to be notoriously expensive because of the high cost associated with reagent consumption. The methods used to process these metals tend to dissolve even silica and alumina, which are frequently the base matrix of platinum-group metals. As a result, the process suffers from high acid consumption and severe acid corrosion problems. Recently, researchers at the U.S. Bureau of Mines have developed a hydrometallurgical process where cyanide is used in an autoclave at high temperatures and pressures. Although the metallurgical efficiency of this process is reasonably good, it suffers from the disadvantage of using toxic cyanide as the major reagent and low recovery of rhodium. Furthermore, this process also suffers from high reagent consumption.
Researchers at the South Dakota School of Mines and Technology (SDSM&T) have recently developed noble technologies of extracting precious metals including PGM from ores and automobile catalytic converters using ammonia and/or halogen salts (Han et al. U.S. Pat. Nos. 5,114,687; 5,308,381; 5,328,669), all of which herein incorporated by reference in their entirety. These processes teach how well precious metals could be recovered from ores and other materials using environmentally benign reagents. However, the recoveries of rhodium and rhenium are not always very satisfactory.
In view of the inadequate recoveries of certain precious metals in the known prior art processes, a need has developed to provide recovery method which overcome the deficiencies of the prior art. In response to this need, the present invention provides an improved method of recovering or extracting precious metals from both primary and secondary sources which yields increased recovery values, especially for hard to recover elements such as rhodium and rhenium. According to the invention, precious metals are recovered using technology similar to the Han et al. patents cited above with the novel and unobvious use of ammonium salts of halogen and one or more of sulfuric or other mineral acids and an ammonium salt.